This invention relates to xerographic development systems and more particularly to the minimization of additives depletion from developer material and the prevention of image surface filming.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
The invention is particularly useful in highlight color imaging such as tri-level imaging. The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught in the '929 patent, the xerographic contrast on the charge retentive surface or photoreceptor is divided into three levels, rather than two levels as is the case in conventional xerography. The photoreceptor is charged, typically to -900 volts. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.cad or V.sub.ddp). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.dad or V.sub.c (typically -100 volts) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically -500 volts) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 volts closer to V.sub.cad than V.sub.white (about -600 volts), and the DAD developer system is biased about 100 volts closer to V.sub.dad than V.sub.white (about -400 volts).
In a tri-level imaging apparatus where the color developer is deposited on the electrostatic images using the DAD developer housing, problems of photoreceptor filming and developer conductivity failures have been experienced. This is because the developer additives provided for maintaining proper developer conductivity and developer flow can be developed on the photoreceptor in the background areas thereby causing photoreceptor filming and depletion of material in the developer which is provided for maintaining proper developer conductivity.
Accordingly, it is a primary purpose of this invention to provide a developer apparatus which minimizes the depletion of certain additives from the developer material contained in the developer apparatus.
It is a more specific purpose of this invention to intercept certain additives contained in the developer prior to developer deposition on the latent image and returning the additives to the developer supply thereby minimizing photoreceptor filming and reduction in developer conductivity.
The following patents relate to techniques for removing various undesirable materials from developer either prior to the developer material being deposited on latent electrostatic images contained on a charge retentive surface or subsequent to such deposition:
U.S. Pat. No. 4,494,863 granted to John R. Lang on Jan. 22, 1985 relates to a toner removal device for removing residual toner and debris from a charge retentive surface after transfer of toner images from the surface. This device is characterized by the use of a pair of detoning rolls, one for removing toner from a biased cleaner brush and the other for removing debris such as paper fibers and Kaolin from the brush. The rolls are electrically biased so that one of them attracts toner from the brush while the other one attracts debris. Thus,the toner can be reused without degradation of copy quality while the debris can be discarded.
U.S. Pat. No. 4,761,668 granted to Parker et al on Aug. 2, 1988 relates to an apparatus for minimizing the contamination of one dry toner or developer by another dry toner or developer used for rendering visible latent electrostatic images formed on a charge retentive surface such as a photoconductive imaging member. The apparatus causes the otherwise contaminating dry toner or developer to be attracted to the charge retentive surface in its inter-document and outboard areas. The dry toner or developer so attracted is subsequently removed from the imaging member at the cleaning station.
U.S. Pat. No. 4,705,387 granted to Ying-wei Lin on Nov. 7, 1987 relates to an apparatus for removing residual charged particles from a charge retentive surface characterized by a particle removal roller and a detoning roller, the former of which is adapted to remove the residual particles from the charge retentive surface and the latter of which removes the particles transferred to the particle removal roller. The detoning roller comprises an array of conductive electrodes extending about the circumference thereof such that when a multi-phase power source is applied thereto a travelling electrostatic wave is generated which causes charged particles having a predetermined diameter and charge to be moved axially to the detoning roller towards one end thereof. The particles so moved represent toner devoid of paper debris. Thus they are suitable for reuse.
U.S. Pat. No. 4,639,115 granted to Ying-wei Lin on Jan. 27, 1987 relates to Apparatus for purifying toner prior to its use in developing latent electrostatic images. An electrically biased roll supported in the developer housing contiguous to at least one of the development rolls serves to attract paper debris from the toner contained in the toner carried by the developer roll. The roll is fabricated from a suitable insulating material and electrically biased in a manner suitable for attracting the paper debris contained in the toner. The roll is rotated and a scraper blade is provided for removing the debris therefrom. The debris so removed is allowed to fall into a catch tray which can be provided with an auger for moving it out of the tray to thereby increase the capacity of the system for debris removal.